A British soldier inside a trench on the Western Front during World War I, 1914–18.

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World War I was one of the great watersheds of 20th-century geopolitical history. It led to the fall of four great imperial dynasties (in Germany, Russia, Austria-Hungary, and Turkey), resulted in the Bolshevik Revolution in Russia, and, in its destabilization of European society, laid the groundwork for World War II.

A collection of significant facts about World War I.

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The outbreak of war

With Serbia already much aggrandized by the two Balkan Wars (1912–13, 1913), Serbian nationalists turned their attention back to the idea of “liberating” the South Slavs of Austria-Hungary. Colonel Dragutin Dimitrijević, head of Serbia’s military intelligence, was also, under the alias “Apis,” head of the secret society Union or Death, pledged to the pursuit of this pan-Serbian ambition. Believing that the Serbs’ cause would be served by the death of the Austrian archduke Franz Ferdinand, heir presumptive to the Austrian emperor Franz Joseph, and learning that the Archduke was about to visit Bosnia on a tour of military inspection, Apis plotted his assassination. Nikola Pašić, the Serbian prime minister and an enemy of Apis, heard of the plot and warned the Austrian government of it, but his message was too cautiously worded to be understood.

Overview of the start of World War I, including details of the June 28, 1914, assassination of …

Austrian archduke Franz Ferdinand and his wife, Sophie, riding in an open carriage at Sarajevo …

Henry Guttmann—Hulton Archive/Getty Images

The Austrians decided to present an unacceptable ultimatum to Serbia and then to declare war, relying on Germany to deter Russia from intervention. Though the terms of the ultimatum were finally approved on July 19, its delivery was postponed to the evening of July 23, since by that time the French president, Raymond Poincaré, and his premier, René Viviani, who had set off on a state visit to Russia on July 15, would be on their way home and therefore unable to concert an immediate reaction with their Russian allies. When the delivery was announced, on July 24, Russia declared that Austria-Hungary must not be allowed to crush Serbia.

Serbia replied to the ultimatum on July 25, accepting most of its demands but protesting against two of them—namely, that Serbian officials (unnamed) should be dismissed at Austria-Hungary’s behest and that Austro-Hungarian officials should take part, on Serbian soil, in proceedings against organizations hostile to Austria-Hungary. Though Serbia offered to submit the issue to international arbitration, Austria-Hungary promptly severed diplomatic relations and ordered partial mobilization.

Home from his cruise on July 27, William learned on July 28 how Serbia had replied to the ultimatum. At once he instructed the German Foreign Office to tell Austria-Hungary that there was no longer any justification for war and that it should content itself with a temporary occupation of Belgrade. But, meanwhile, the German Foreign Office had been giving such encouragement to Berchtold that already on July 27 he had persuaded Franz Joseph to authorize war against Serbia. War was in fact declared on July 28, and Austro-Hungarian artillery began to bombard Belgrade the next day. Russia then ordered partial mobilization against Austria-Hungary, and on July 30, when Austria-Hungary was riposting conventionally with an order of mobilization on its Russian frontier, Russia ordered general mobilization. Germany, which since July 28 had still been hoping, in disregard of earlier warning hints from Great Britain, that Austria-Hungary’s war against Serbia could be “localized” to the Balkans, was now disillusioned insofar as eastern Europe was concerned. On July 31 Germany sent a 24-hour ultimatum requiring Russia to halt its mobilization and an 18-hour ultimatum requiring France to promise neutrality in the event of war between Russia and Germany.

Both Russia and France predictably ignored these demands. On August 1 Germany ordered general mobilization and declared war against Russia, and France likewise ordered general mobilization. The next day Germany sent troops into Luxembourg and demanded from Belgium free passage for German troops across its neutral territory. On August 3 Germany declared war against France.

In the night of August 3–4 German forces invaded Belgium. Thereupon, Great Britain, which had no concern with Serbia and no express obligation to fight either for Russia or for France but was expressly committed to defend Belgium, on August 4 declared war against Germany.

Austria-Hungary declared war against Russia on August 5; Serbia against Germany on August 6; Montenegro against Austria-Hungary on August 7 and against Germany on August 12; France and Great Britain against Austria-Hungary on August 10 and on August 12, respectively; Japan against Germany on August 23; Austria-Hungary against Japan on August 25 and against Belgium on August 28.

Romania had renewed its secret anti-Russian alliance of 1883 with the Central Powers on February 26, 1914, but now chose to remain neutral. Italy had confirmed the Triple Alliance on December 7, 1912, but could now propound formal arguments for disregarding it: first, Italy was not obliged to support its allies in a war of aggression; second, the original treaty of 1882 had stated expressly that the alliance was not against England.

On September 5, 1914, Russia, France, and Great Britain concluded the Treaty of London, each promising not to make a separate peace with the Central Powers. Thenceforth, they could be called the Allied, or Entente, powers, or simply the Allies.

The outbreak of war in August 1914 was generally greeted with confidence and jubilation by the peoples of Europe, among whom it inspired a wave of patriotic feeling and celebration. Few people imagined how long or how disastrous a war between the great nations of Europe could be, and most believed that their country’s side would be victorious within a matter of months. The war was welcomed either patriotically, as a defensive one imposed by national necessity, or idealistically, as one for upholding right against might, the sanctity of treaties, and international morality.

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World War I lasted from 1914 to 1918. It was known at first as the Great War and the War to End All Wars. It was the largest war that the world had seen up to that time. Most of the battles took place in Europe and the Middle East. More than 8 million soldiers and sailors died, and more than 20 million were injured. The war was so bad that many people thought no one would ever start another war. However, the results of World War I helped to cause World War II, which broke out in 1939.

A major international conflict fought from 1914 to 1918, World War I was the most deadly and destructive war the world had ever seen to that time. More than 25 countries eventually participated, aligned with either the Allied or the Central powers. The Allies-who won the war-included primarily France, Great Britain, Russia, Italy, Japan, and, from 1917, the United States. The Central Powers consisted mainly of Germany, Austria-Hungary, and the Ottoman Empire (Turkey). World War I felled four great imperial dynasties, in Germany, Russia, Austria-Hungary, and Turkey. It led to revolution in Russia, destabilized Europe, and laid the foundations for World War II. (For a chronology of events for World War I, see World War I Chronology.)